linux-kernel - Re: [PATCH v15 09/12] dm: Add support for copy offload

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Message-ID: <20230911070724.GA28177@green245>
Date:   Mon, 11 Sep 2023 12:37:24 +0530
From:   Nitesh Shetty <nj.shetty@...sung.com>
To:     Hannes Reinecke <hare@...e.de>
Cc:     Jens Axboe <axboe@...nel.dk>, Jonathan Corbet <corbet@....net>,
        Alasdair Kergon <agk@...hat.com>,
        Mike Snitzer <snitzer@...nel.org>, dm-devel@...hat.com,
        Keith Busch <kbusch@...nel.org>,
        Christoph Hellwig <hch@....de>,
        Sagi Grimberg <sagi@...mberg.me>,
        Chaitanya Kulkarni <kch@...dia.com>,
        Alexander Viro <viro@...iv.linux.org.uk>,
        Christian Brauner <brauner@...nel.org>,
        martin.petersen@...cle.com, mcgrof@...nel.org,
        gost.dev@...sung.com, linux-block@...r.kernel.org,
        linux-kernel@...r.kernel.org, linux-doc@...r.kernel.org,
        linux-nvme@...ts.infradead.org, linux-fsdevel@...r.kernel.org
Subject: Re: [PATCH v15 09/12] dm: Add support for copy offload

On Fri, Sep 08, 2023 at 08:13:37AM +0200, Hannes Reinecke wrote:
> On 9/6/23 18:38, Nitesh Shetty wrote:
> > Before enabling copy for dm target, check if underlying devices and
> > dm target support copy. Avoid split happening inside dm target.
> > Fail early if the request needs split, currently splitting copy
> > request is not supported.
> > 
> And here is where I would have expected the emulation to take place;
> didn't you have it in one of the earlier iterations?

No, but it was the other way round.
In dm-kcopyd we used device offload, if that was possible, before using default
dm-mapper copy. It was dropped in the current series,
to streamline the patches and make the series easier to review.

> After all, device-mapper already has the infrastructure for copying
> data between devices, so adding a copy-offload emulation for device-mapper
> should be trivial.
I did not understand this, can you please elaborate ?

Thank you,
Nitesh Shetty